1. Field of the Invention
The present invention relates to a coil-embedded dust core, which may be used in inductors having a unitary structure with a magnetic core and in other electronic components. The present invention also relates to a method for manufacturing the coil-embedded dust core. More particularly, the invention relates to a method for manufacturing coil-embedded dust core constructed by embedding an air-core coil in a green body, and the like.
2. Description of the Related Art
In recent years, electric and electronic equipment has become more compact, and dust cores that are compact (low in height) yet able to accommodate large current have come to be in demand.
Materials used for dust cores are ferrite powder and ferromagnetic metal powder, but ferromagnetic metal powder has larger saturation magnetic flux density than ferrite powder and its DC bias characteristics may be maintained even in a strong magnetic field. Consequently, in making a dust core that can accommodate large current, using ferromagnetic metal powder as a material for dust core has become mainstream.
In addition, in order to further the effort to make the core more compact (lower in height), a coil body in which a coil and compacted magnetic powder form a unitary structure has been proposed. In the present specification, an inductor having such a structure may be called a “coil-embedded dust core.”
A manufacturing method for a surface-mount type inductor having a structure of a coil-embedded dust core has been proposed in the past. For example, Japanese Patent Laid-Open No. 5-291046 discloses that an exterior electrode is connected to an insulation-coated conduction wire, and these are enclosed in magnetic powder, which is then compressed into a magnetic body. Japanese Patent Laid-Open No.11-273980 discloses that a composite material made by mixing flat soft magnetic metal powder and binder, and a coil are inserted into a die constituted by a die set and a bottom punch at the same time and compression-forming is performed. Japanese Patent No. 2958807 discloses a method for manufacturing an inductor by compressing magnetic powder while orientating the easy axis of magnetization of magnetic powder along the orientation of the magnetic field formed by energizing the coil in order to obtain a large inductance value. Further, Japanese Patent No.3108931 discloses the method for manufacturing an inductor by preparing a first green body and a second green body which are preformed by compression respectively and by performing main compression-forming until an interface between the first green body and the second green body is removed with the coil being vertically sandwiched with these green bodies. According to the method described in Japanese Patent No. 3108931, a loading weight of magnetic powder constituting the green body can be increased, and therefore a larger inductance value can be obtained than in the above-described Japanese Patent Laid-Open No. 5-291046, Japanese Patent Laid-Open No. 11-273980, and Japanese Patent No. 2958807.
However, according to the method described in Japanese Patent No. 3108931, three forming operations are required, that is, performing of the first green body, performing of the second green body, and main forming performed with the coil being sandwiched by the first green body and the second green body. When these forming operations are performed with one die machine, a die has to be replaced for each forming operation, which is inefficient. Further, in the case of main forming, compressing pressure is increased so that the interface of the preformed core is not left, which causes problems of deformation of the coil, an insulation failure and the like.
As described above, a large inductance value can be obtained with the coil-embedded dust core of a small size, and while size reduction of electric and electronic devices are rapidly advancing, there is a strong demand for improvement in quality of the coil-embedded dust core. In concrete, as the frequency at which the coil-embedded dust core is used shifted to a higher frequency side, the demand for precision of the inductance values increases. Since impedance increases in proportion to a frequency, the coil-embedded dust core has to be designed so that the inductance value decreases as the frequency, at which the coil-embedded dust core is used, is shifted to a higher frequency side. Meanwhile, it is necessary to avoid the situation in which part of the magnetic body is saturated magnetically and a predetermined inductance value (design value) cannot be obtained. Namely, it is required to obtain an inductance value previously specified based on the working frequency with stability.
Thus, in view of the above-described points, the present invention has its object to provide a method for efficiently manufacturing a coil-embedded dust core which attains a predetermined inductance value (design value) with a small variation in inductance value, and the like.